1. Field of the Invention
The present invention relates to a process for treating polyamides, such as nylon, to improve a property of the polyamide. The invention also relates to polyamides having improved characteristics, such as improved dyed color depth, dyed color uniformity, hue, elimination of light dyeing ends, protection of dye sites from degradation, protection against ultra-violet (UV) degradation, reduced yellowing or oxidation, and/or resistance to loss of dyeability.
2. Description of Related Art
Polyamides often contain terminal primary amines, which are affected by reactive and/or oxidizing environments found in prolonged storage or in certain preparatory processing applied to polyamides, such as heat setting or molding.
Heat setting is often applied to substrates containing polyamide. Heat setting involves heat exposure for limited times at limited temperatures. Heat setting is useful, for example, to process polyamide fabrics or garments containing spandex fiber. Heat setting is performed to confer dimensional stability and wrinkle resistance to the fabrics or garment.
Often the heat set process results in some oxidation and yellowing of the nylon with loss of dyeability, dyed color uniformity, color depth, and/or hue. Yellowing of the polyamide component is noticeable in the natural or un-dyed state. Yellowing is objectionable in fabrics meant to be white and detracts from the desired color contrast sought in dyed fabric prints on a white fabric background.
Current methods to impart whiteness retention in nylon fabrics include adding stabilizers of the IRGANOX.RTM. type from CIBA GEIGY (Basel, Switzerland) or Nylostab Seed.RTM. from CLARIANT (Basel, Switzerland). Known whiteness retention and heat stabilizers for nylon include the phosphorus oxy acid type and their salts, as well as, the classes of materials disclosed in U.S. Pat. Nos. 3,839,392 and 3,907,746.
Stabilizers added to nylon polymer often excessively foam the polymer when added to an autoclave polymerization process. Moreover, high stabilizer loadings often produce poor polymer and poor uniformity from batch to batch. Also, loss of the stabilizer additive from nylon yarn and fabrics during aqueous processing steps, especially dyeing, often make the addition of stabilizers undesirable.
There is thus a need to provide polyamides and products, such as fabrics constructed therefrom that retain their dyeability characteristics and whiteness retention, and avoid or reduce disadvantages of current methods.